Hi Joyca,
Yes. Retrofit Transport should be already created and be assigned to the person who is performing the Retrofit Task. He/She should also have Developer Access Key as well for ABAP activation during the Retrofit process execution.
Regarding the Retrofit Transport Creation/Ownership, you can follow this approach:
1. Create a Normal Change and assign it to the Current Implementation Project.
2. Create one Work bench Transport and one Custom Transport per System which can be used during the Retrofit Process.
Please note this approach works well if the number of Transports to be retrofitted are minimal.
Please check the aforementioned link for additional information on Retrofit.
Regards,
Prasath